Due at least in part to the discovery that Er-doped silica optical fiber amplifiers are advantageously pumped with 0.98 .mu.m radiation, and that it would be advantageous to pump Pr-based fluoride fiber amplifiers with radiation of about 1.01 .mu.m wavelength, there is now considerable commercial interest in radiation sources for the approximate 0.87-1.1 .mu.m wavelength regime.
For practical reasons pump lasers for optical fiber amplifiers almost invariably will be semiconductor lasers. It is well known that InP-based lasers can have emission wavelengths .gtoreq.0.94 .mu.m. However, InP-based lasers that have wavelengths in the short-wavelength portion (typically .ltoreq.1 .mu.m) of the wavelength regime accessible to such lasers, typically have some disadvantageous features that make such lasers a poor choice for applications such as optical fiber amplifiers with pump radiation in the range 0.94 .mu.m to about 1 .mu.m. For instance, InP-based lasers that emit in that wavelength range typically have high threshold current and low quantum efficiency, due to their inherently low bandgap difference between InP and the relevant quaternary or ternary semiconductor alloy. On the other hand, the bandgap difference between GaAs and the relevant alloy is relatively large, making possible design of GaAs-based lasers having advantageous properties, e.g., relatively low threshold current and relatively small temperature dependence. Thus, it would be highly desirable to have available a GaAs-based semiconductor laser that can emit at a predetermined wavelength in the approximate wavelength regime 0.87-1.1 .mu.m.
However, as is well known, the bandgaps of GaAs and the relevant alloys are such that all GaAs-based lattice matched lasers will have an emission wavelength that is .ltoreq.0.87 .mu.m.
GaAs-based strained layer QW lasers that emit in the 0.87-1.1 .mu.m wavelength regime are known. See, for instance, M. Okayasu et al., Electronics Letters, Vol. 25(23), p. 1563 (1989); H. K. Choi et al., Applied Physics Letters, Vol. 57(4), p. 321 (1990). However, these prior art lasers are ridge waveguide, buried heterostructure, capped heterostructure, or other difficult to manufacture (typically index-guided) structures that typically require one or more critical alignment steps during their manufacture. A further manufacturing problem associated with many of these prior art lasers is the use of Al-containing semiconductor material. It is well known to those skilled in the art that Al-containing semiconductor alloys have a strong tendency to oxidize, necessitating, for instance, oxide-removal steps prior to re-growth. This is a particular problem when using molecular beam epitaxy (MBE), an otherwise very advantageous growth method.
A laser having a self-aligned structure is typically more easily manufactured than one that is not self-aligned, and self-aligned lasers are known. See, for instance, M. Nido et al., IEEE Journal of Quantum Electronics, Vol. QE-23(6), p. 720 (1987); H. Tanaka et al., Japanese Journal of Applied Physics, Vol. 24(2), p. L89 (1985); and H. Tanaka et al., Journal of Crystal Growth, Vol. III p. 1043 (1991). Prior art GaAs-based self-aligned lasers have bulk current blocking layers that are lattice matched to the substrate, and their emission wavelength consequently is determined solely by the bandgap energy of the active layer material. It is for this reason that such lasers cannot be designed to emit in the above referred-to wavelength regime, since there are no suitable GaAs-based alloys that have E.sub.g corresponding to a wavelength in that regime. Furthermore, the prior art GaAs-based self-aligned lasers typically involve Al-containing semiconductor alloy, and thus require special steps during manufacture. For instance, H. Tanaka et al. (op. cit.) disclose a manufacturing process that comprises chemically patterning and thinning a GaAs layer such that only a 100 nm thick passivation layer remains, followed by thermal desorption of the passivation layer just before the start of the second epitaxial growth cycle.
In view of the above discussion it is evident that it would be highly desirable to have available a GaAs-based, self-aligned (and thus readily manufacturable) semiconductor laser whose emission wavelength is in the approximate range 0.87-1.1 .mu.m. This application discloses such a laser.